Removal of CO2 and other greenhouse gasses (“GHG”) from the atmosphere by sequestration within deep geological formations has long been seen as a potential solution to the problems presented by the buildup of these gasses in the atmosphere. Typically, a gas mixture that is rich in a greenhouse gas component is injected into an aquifer such that the gas mixture or the greenhouse gas component thereof dissolves into and is sequestered within the aquifer. The gas mixture may comprise effluent from a fossil-fuel burning plant or other source, optionally enriched in its greenhouse gas component.
Unfortunately, efforts to economically sequester GHG on a commercial scale have thus far been elusive. One issue has been the relatively slow speed at which gasses are absorbed and trapped within a formation using conventional injection techniques. In particular, in conventional processes the front that arises of the injected gas can be slow to advance within the formation, leading to a slow rate of mixing and dissolution of the gasses into the formation water.
Injection of a non-aqueous phase into a water-wet saturated porous medium is a process that appears many times in energy resource extraction (gas injection, solvent injection), hydrogeology (gasoline leakage, air injection, contamination by dense non-aqueous petroleum liquids such as chlorinated hydrocarbon), environmental engineering (such as air sparging for site cleanup), and chemical process engineering (such as operation of fabricated porous reaction beds perhaps containing solid catalysts). The physical process of multiphase flow is usually divided into simultaneous mechanical and chemical interactions of the fluids and the solid skeleton, depending on the nature of the problem being addressed.
Carbon Capture and Sequestration (CCS) is a candidate method to control GHG concentrations in the atmosphere, thereby reducing the magnitude of the predicted effects, and ideally CCS would help to stabilize and eventually even reduce the atmospheric CO2 concentration level. One approach to CCS involves the separation of CO2 from a gaseous stream such as flue gas from a coal-fired power plant, a carbon dioxide enriched stream from novel methods of coal combustion, from cement kilns, from ammonia plants or other chemical manufacturing plants, or from other point sources. The separated CO2 is purified to a level where it can be injected deep into a porous saline aquifer. In the past, it has been generally assumed that the CO2 must be essentially pure so that it can be injected in the supercritical state (a compressible liquid of low viscosity and moderate density—0.60-0.75 g/cm3). Herein, super-critical CO2 is abbreviated as SC-CO2. Capturing the CO2 component from the point source, compression and transportation of CO2, and finally depositing it into a secure subsurface formation, are the general parts of various CCS systems.
CCS by injection into deep geological formations is a potential disposal (storage) method. Technology for deep injection of gaseous or supercritical phases exists in oil and gas industries, though injection process optimization using horizontal wells, cyclic injection of water and CO2, trap security and long-term risk, and related subjects remain of research interest (Bachu et al., 1994; Stevens et al., 2008).
Because of the lower density compared to the saline water present in deep aquifers, a buoyancy effect is an intrinsic characteristic property of the injected CO2-enriched gas and remains associated with the accumulation of the injectate as a free phase near the top of the saline aquifer. The late time configuration after years to decades of injection is a thick overlying zone of the buoyant materials, which gives a permanent risk of leakage from the cap rock or any sealing formation. This free cap of buoyant CO2 as a gas or as SC-CO2 remaining in place for hundreds to thousands of years remains a major concern for all saline aquifer sequestration methods that involve a discrete upper buoyant CO2 accumulation.
Even though several partial barriers to upward movement may exist, as long as there is a thick cap of the less dense phase, there is the possibility of leakage leading to the eventual escape of the sequestered GHG.
Hydrodynamic trapping mechanisms are considered as the least secure and the least reliable isolation alternative, in contrast to dissolution processes where the CO2 is dissolved in the aqueous phase, solidification of the injected CO2 in terms of generating solid mineral precipitates through reactions at depth (likely impractical), or the direct injection of carbon-rich solid matter such as petcoke (petroleum coke), biosolids or other biological wastes.